Amplitude detection scheme for optical transmitter control

Abstract: An amplitude control scheme for a high bit rate digital optical transmitter is disclosed. The data to be transmitted by the laser is pulse-width modulated by a low frequency signal (MOD). The pulse-width modulated signal is applied to the laser via a laser driver (14) and to a mark density reference generator (15). The magnitude of the low frequency components from the mark density reference generator is a signal (ARS) indicative of the desired amplitude of the laser light pulses. A back-face photodiode (20) converts a portion of the laser light into an electrical signal, the magnitude of the low frequency portion thereof being a signal (ACS) indicative of the actual amplitude of the laser light pulses. The actual amplitude of the laser light pulses is compared to the desired amplitude and the laser driver output amplitude may then adjusted to compensate for variations in the laser performance.

Description: This invention relates to digital lightwave transmitters and to methods for monitoring and controlling the amplitude of a digital laser light output signal. A continuing concern in the field of laser-based optical transmitters is the change in laser characteristics with temperature and aging. The change in laser characteristics manifests itself as a combination of a shift in the laser threshold, (the current at which the laser begins lasing), and the L-I slope (the light output L vs. current I characteristic). As a result, special operating strategies for controlling the laser bias and modulation currents have been developed. In most cases, a photodiode is mounted in the same package as the laser and a portion of the light exiting the rear face of the laser is captured by the photodiode and used to monitor the laser performance. In particular, a feedback loop comparing the photodiode current to a reference signal maintains the average light output at a desired level by automatically adjusting the bias current. The reference signal is referred to here as a "mark density" reference signal, representing the average density over time of pulses which the laser converts into light. During initial use, and as long as only the laser threshold exhibits changes as a function of aging, the above photodiode monitor arrangement is suitable. However, when the laser L-I slope begins to change as a function of age, the light amplitude (and with it, the critical extinction ratio of ON-light to OFF-light) will change. Thus, a need remains for a means of controlling both the average light output and the light amplitude. Further, prior art arrangements work only with lasers having an exceptionally linear L-I slope, irrespective of temperature or aging. Typical lasers, however, have non-linear L-I slope which may render this approach unusable. Thus, a need remains for a more robust arrangement for monitoring both average light and light amplitude in an optical transmitter that is usable with non-ideal lasers. The present invention seeks to provide control of the laser modulation over the lifetime of the optical transmitter. According to one aspect of this invention there is provided a digital lightwave transmitter as claimed in claim 1. According to another aspect of this invention there is provided a method as claimed in claim 6. The invention will now be described with reference to the accompanying drawings, in which: FIG. 1 illustrates an exemplary laser transmitter embodying the present invention; FIG. 2 is a simplified schematic diagram of the laser driver of FIG. 1; FIG. 3 is a simplified schematic diagram of the mark density reference generator of FIG. 1; and FIG. 4 is a plot (not to scale) of a light amplitude control signal measured against the laser light pulse amplitude for different modulation depths in one exemplary embodiment of the invention. Detailed Description An exemplary digital lightwave transmitter 10 utilizing both average light and low frequency light amplitude control is illustrated in FIG. 1. In general, an incoming digital data signal (electrical) D, operating at a predetermined bit rate is coupled into the low frequency light amplitude control means 12. Low frequency means 12 will be discussed in more detail below. Data signal D subsequently passes through a laser driver 14 and is applied as the modulation input to a laser device 16. A bias current I bias is also applied as an input to laser device 16. The light output signal from laser device 16 may then be coupled into an optical fiber (not shown) to propagate along the signal path as a digital optical data signal. As seen in FIG. 1, a portion of the optical output signal from laser device 16 also exits the rear face of the device and illuminates a monitor photodetector 20, here a photodiode. The optical signal is converted into an electrical current within monitor photodetector 20, denoted as I MON . As is well-known in the art, I MON may be applied as a first input to a control means 22 for monitoring the average light output signal from laser device 16. As described above, I MON may be compared to the mark density level of the electrical pulses to the laser driver 14, a substantially DC signal, to provide any necessary adjustments to the level of I BIAS to maintain the desired average light output. It is the control means 12, as will be described in greater detail hereinafter, that provides the additional ability to monitor and control the amplitude of the laser output signal. As illustrated in the exemplary embodiment of FIG. 1, an exemplary high bit rate digital data stream D is being supplied as an input to control means 12. The stream D is applied as a first input to pulse width modulator 30. Data stream D is then pulse width modulated by a low frequency signal MOD from a low frequency oscillator 32 to form a pulse-width modulated data output signal D PWM . The signal D PWM drives laser driver 14 which, in turn, drives the laser diode 16. As a consequence, the laser light pulses from diode 16 are also pulse-width modulated For purposes of this discussion, the term "low frequency" refers to a signal having a frequency significantly lower than the frequency of the digital data that it modulates in pulse width modulator 30. An illustrative laser driver 14 is shown in FIG. 2. The driver 14 allows the laser light pulses to vary in amplitude depending on the current supplied by the current source 50, as will be discussed below. The pulse-width modulated data signal D PWM additionally drives a mark density reference generator 15 which generates a signal indicative of the density of the pulses applied to the laser 16. An exemplary generator 15 is shown in FIG. 3 and is similar to the laser driver 14 of FIG. 2 but with fixed bias (tail) current. Returning to FIG. 1, the signal from mark density reference generator 15 has two components, a DC component (MDS DC ) representing the average level of the pulses and an AC component (MDS AC ) representing, for purposes here, the effect of the pulse-width modulation on the data signal D. The DC component MDS DC is coupled to the laser bias control 22, discussed above. The AC component MDS AC is amplified by a low frequency AC-coupled amplifier 36 and demodulated to form an amplitude reference signal (ARS), as discussed below in more detail, since the signal to the mark density reference generator 15 has not yet passed through the laser 16. The ARS signal represents the desired amplitude of the laser light pulse. As stated above, the laser light pulses from laser 16 will have the low frequency pulse width modulation impressed thereon. Monitor photodetector (photodiode) 20 will, therefore, re-convert this low frequency signal into an electrical equivalent which may then be evaluated to determine any change in the light amplitude of the laser output signal. In particular, the output signal from monitor photodetector 20 will contain both DC and AC components, the DC component (I MON - DC) is related to the bias current, as described above. Therefore, the AC component I MON - AC may be amplified in a manner similar to the AC component of the mark density reference signal MDS AC , utilizing a second low frequency AC-coupled amplifier 38 (wherein amplifier 38 should be essentially matched in performance to first amplifier 36). The signal I MON - AC is subsequently demodulated to generate an amplitude control signal, ACS. The signal ACS is indicative of the amplitude of the actual laser output signal. In accordance with the present embodiment, this amplitude signal may be controlled by comparing the actual output signal ACS to the generated reference signal ARS within a light amplitude controller 42 to produce an error signal E. The output error signal E from controller 42 will provide an indication of any difference between these two signals and serves as a control signal to laser driver 14 to either increase or decrease the amplitude of the driver signal applied to the laser. Referring temporarily back to FIG. 2, the error signal E from controller 42 varies a current source 50 to adjust the amplitude of the current through the laser 16 and, hence the amplitude of the laser light output pulse therefrom. Therefore, by applying pulse width modulation to the laser modulating current, and detecting the low frequency component in a monitor photodiode, a relative measure of the light amplitude may be obtained, and any changes in this amplitude be corrected. The signals ARS and ACS are essentially indicative of the amplitude of the low frequency signals from corresponding amplifiers 36, 38. As such, the detectors may be conventional envelope detectors. However, it has been found that a conventional envelope detector is unsuitable due to the low signal levels and high noise environment within a laser transmitter package, i.e, the low frequency signals have a low signal-to-noise ratio. To improve the signal-to-noise ratio of the demodulation of the low frequency signals, it is preferred that synchronous demodulation be used to detect the modulating signal in the D PWM and I MON - AC signals. Thus, synchronous demodulators 34, 40, here shown as multipliers, utilize the low frequency modulation signal MOD as the demodulation reference signal. It is noted that the low frequency oscillator 32 may produce a square wave, a sinusoidal wave, triangular wave, etc. or may be a random or pseudorandom signal. The frequency of the oscillator 32 should be lower than the data D bit rate, preferably two orders of magnitude or more lower. To confirm the above exemplary embodiment, portions of a digital laser transmitter have been fabricated and tested using a GaAs photodiode back-face monitor with a 1.3µm multimode laser. The low frequency oscillator used was a 5 KHz square wave oscillator, pulse-width modulating a 1.1 Gb/s digital data signal at different modulation depths to produce data pulse width variations of 8, 25 and 80 ps. As shown in FIG. 4, the amplitude control signal (ACS, FIG. 1) was measured and was found to be linear with light amplitude from the laser 16. Further, the entire transmitter has been simulated using a data rate of 10 Mbits/sec and a low frequency modulation rate of 100 KHz. The laser was simulated to shift its threshold current and L-I slope by approximately 17 and 22 percent, respectively, while operating "closed loop" under the control of the light amplitude control means 12 (FIG. 1). The simulations verify that the means 12 does control the light amplitude from the laser 16 with the different L-I slopes and threshold shifts.

Claims: A digital lightwave transmitter comprising a laser diode (16) responsive to both a digital modulation signal and a bias signal for forming an optical digital output signal therefrom; a monitor photodetector (20) coupled to the back-face of the laser diode for coupling a portion of the optical digital output signal and deriving an electrical control signal I MON therefrom; means (14) for driving the laser diode with the digital modulation signal; and CHARACTERIZED BY: a low frequency oscillator (32), for producing a modulating signal at a predetermined low frequency; a pulse width modulator (30) responsive to both an input digital data signal and the low frequency modulating signal for providing as an output a pulse width modulated digital data signal, wherein the pulse width modulated digital data signal is applied as an input to the means for driving the laser diode; means (15, 34) for extracting the AC component of the pulse width modulated digital signal and multiplying the AC component with the low frequency modulating signal to form a light amplitude reference signal (ARS); means (40) for extracting the AC component of the monitor photodiode output signal and multiplying the AC component with the low frequency modulating signal to form a light amplitude control signal (ACS) indicative of the amplitude of the digital output signal; and means (42) for comparing the light amplitude reference signal to the light amplitude control signal and providing an output control signal indicative of any difference between the light amplitude signals, wherein the output control signal is applied as an input to the driving means to adjust the amplitude of the pulse width modulated digital data signal applied as an input thereto. A transmitter as claimed in claim 1 comprising means (22) for comparing the DC component of the pulse width modulated digital signal to the DC component of the monitor signal and for providing as an output a control signal to the bias signal, indicative of any difference in the values of the DC inputs. A transmitter as claimed in claim 2 wherein a first low frequency amplifier (36) is used to amplify the AC component of the pulse width modulated digital data signal, and a second low frequency amplifier (38) is used to amplify the AC component of the monitor signal. A transmitter as claimed in claim 3 wherein the low frequency oscillator provides as an output a square wave low frequency signal. A transmitter as claimed in claim 3 wherein the low frequency oscillator provides as an output a sine wave low frequency signal. A method for monitoring and controlling the amplitude of a digital laser light output signal from a digital laser transmitter having a laser (16) responsive to a laser driver (14) and a back-face photodetector (20) for converting at least part of the light from the laser into a monitor signal, CHARACTERIZED BY THE STEPS OF: pulse width modulating a high bit rate digital data input signal with a low frequency modulation signal to form a pulse width modulated data signal which is coupled to the laser driver; extracting the low frequency AC component of the monitor signal to form a laser amplitude control signal (ACS); extracting the low frequency AC component of the pulse width modulated data signal to form a reference signal (ARS); and comparing said laser amplitude control signal to the reference signal and providing as an output an amplitude error signal (E) indicative of any difference in value between the laser control and reference signals; wherein the laser driver includes a control input for varying the drive to the laser and the control input is responsive to the amplitude error signal. A method as claimed in claim 6 comprising the steps of: comparing the DC component of the pulse width modulated digital data signal to the DC component of the monitor signal, and providing an amplitude control signal, indicative of the difference therebetween, as a bias control signal (I bias ) to the laser. A method as claimed in claim 7, wherein the extracting step forming the laser amplitude control signal comprises filtering the monitor signal to pass the low frequency AC component thereof; and demodulating the low frequency AC component; wherein the demodulated low frequency AC component is the amplitude control signal. A method as claimed in claim 8, wherein the extracting step forming the reference signal comprises measuring the mark density of the pulse width modulated data signal to form a mark density signal (MDS AC ); filtering the mark density signal to pass the low frequency AC component thereof; and demodulating the low frequency AC component; wherein the demodulated low frequency AC component is the reference signal. A method as claimed in claim 8 wherein the low frequency modulation signal is a square wave. A method as claimed in claim 8 wherein the low frequency modulation signal is a sine wave.

Inventors: BOSCH FRIDOLIN LUDWIG, NGUYEN TON VAN, BOSCH, FRIDOLIN LUDWIG, NGUYEN, TON VAN
Applicants: AT & T CORP, AT&T CORP.
Application year: 1994
Main classification: H01S3
IPC codes: H01S3, H04B10, H01S5